Recently, greater capacity and higher speed of wireless communication are increasingly demanded, and a research upon methods of improving the effective use efficiency of limited frequency resources is popular. As one of these methods, a method of utilizing the spatial domain is focused upon. Adaptive array antennas (adaptive antennas) are one of spatial domain utilizing techniques.
This technique adjusts amplitude and phase with a weighting coefficient (hereinafter this weighting coefficient will be referred to as “weight”) multiplied upon a received signal. By this means, a desired signal arriving from a desired direction can be received with strong power, so that it is possible to cancel interference component signals such as multipath interference and co-channel interference. As a result of such interference canceling effect, communication capacity of a communication system can be improved.
Further, other techniques utilizing the space domain include a spatial-multiplexing technique of transmitting different data sequences to the same terminal apparatus using physical channels of the same time, the same frequency and the same code by utilizing spatial orthogonality in the channel. The spatial-multiplexing technique is disclosed in, for example, Non-Patent Document 1.
According to the spatial-multiplexing technique, a plurality of antenna elements are provided in a transmitter and a receiver. Further, if the spatial-multiplexing technique is used, it is possible to realize spatial-multiplexing transmission under an environment where correlation between received signals of antennas is little.
In this case, different data sequences are transmitted using physical channels of the same time, the same frequency and the same code between antenna elements, from a plurality of antennas provided in the transmitter. The receiver demultiplexes received signals received at a plurality of antennas based on channel characteristic estimation values.
By this means, higher speed is realized by using a plurality of spatial-multiplexing channels without using M-ary modulation. When spatial-multiplexing transmission is carried out, under an environment where there are a plurality of scatterings between a transmitter and a receiver with a condition where the S/N (signal to noise ratio) is sufficient, if the transmitter and the receiver have the same number of antennas, it is possible to increase communication capacity in proportion to the number of antennas.
When such spatial-multiplexing transmission is carried out, a multicarrier modulation scheme using orthogonal frequency division multiplexing (OFDM) is generally used because, in the multicarrier modulation scheme, if the multipathdelay of a radio channel is within the guard interval time, channel fluctuation that influences upon each subcarrier is regarded as flat fading, and so multipath equalization processing is not necessary and processing of demultiplexing signals subjected to spatial-multiplexing transmission is reduced.
Here, the multicarrier modulation scheme is a transmission scheme using a plurality of subcarriers. The input data signal to each subcarrier is modulated using M-ary QAM modulation and the like, and a subcarrier signal is obtained. OFDM refers to a multicarrier modulation scheme where frequencies between subcarriers are orthogonal. Further, OFDM refers to collectively converting subcarrier signals of different frequencies into time domain signals using a fast Fourier transform circuit, carrying out a frequency conversion on the time domain signals into the carrier frequency band and transmitting these signals from antennas.
On the other hand, a receiving apparatus carries out a frequency conversion on the signals received at antennas into baseband signals and carries out OFDM demodulation processing. Upon such a frequency conversion operation, phase noise is included in the received signals. Although the carrier frequency difference between a transmitter and a receiver can be reduced by an automatic frequency control (AFC) circuit, the residual carrier frequency difference, which is the component of the carrier frequency difference, remains. When M-ary value QAM is used to modulate subcarriers, data decision is carried out upon demodulation with the absolute phase as a reference using a detection circuit, and so the residual carrier frequency difference remains or detection error occurs upon phase rotation due to phase noise and therefore reception characteristics deteriorate.
As a method of compensating for such phase rotation, phase tracking is generally carried out to transmit a known pilot subcarrier signal from a transmitting apparatus, detect the amount of phase rotation of a pilot subcarrier (PSC) in a receiving apparatus and carry out phase compensation based on this detection result. Further, in the following description, the pilot subcarrier is simply referred to as “PSC.”
FIG. 1 shows a configuration example of a transmission frame including pilot subcarrier signals. As shown in FIG. 1, the transmission frame is configured with training signal portion A1, signaling portion A2 and data portion A3. Further, data portion A3 includes pilot subcarrier signal (PSC signals) A4 in a specific subcarrier.
FIG. 2 shows a configuration of a wireless communication apparatus including a phase tracking circuit disclosed in Patent Document 1. In FIG. 2, the wireless communication apparatus carries out the following receiving operation with respect to signals of the transmission frame configuration as shown in FIG. 1 which are transmitted after OFDM modulation. First, automatic gain control (AGC) is carried out in AGC section B1 using a received signal of training signal portion A1, and therefore the received signal level is adjusted to be adequate. Then, the frequency difference is corrected by carrying out automatic frequency control (AFC) in AFC section B2, and FFT processing is carried out in FFT section B3. Then, channel equalizing section B4 calculates a channel estimation value showing a channel fluctuation and carries out channel equalization processing. Then, the signal of signaling portion A2 is detected. The signal of signaling portion A2 includes information of the coding rate of the error correction code and the M-ary modulation value.
Next, subcarrier phase tracking circuit B5 receives the signal of data portion A3 subjected to channel equalization as input and carries out the following operation. First, PSC signal extracting section B6 extracts PSC signal A4 from the equalized subcarrier signal of data portion A3. Then, phase rotation detecting section B7 detects the phase rotation of the subcarrier signal after channel equalization, based on extracted PSC signal A4 and a replica signal of the PSC signal. Phase compensating section B8 compensates for the detected phase rotation with respect to the subcarrier signal of data portion A3 subjected to channel equalization, and outputs the signal to subsequent decoding section B9.
Based on information obtained in signaling portion A2, that is, based on coding modulation information of a transmission stream, decoding section B9 carries out demapping processing for converting a symbol data sequence modulated according to a predetermined modulation scheme into a bit sequence, deinterleaving processing which is reverse processing of interleaving processing on the transmitting side and error correction decoding processing with respect to the bit data sequence, thereby restoring the transmission bit sequence.
By this means, it is possible to compensate for the residual carrier frequency difference due to AFC error or the phase rotation, which changes over time, due to the sampling clock difference in the analogue-to-digital converter (A/D), using phase tracking circuit B5. That is, phase compensation can be carried out following the phase rotation, so that it is possible to carry out synchronous detection stably. By this means, received quality in a wireless communication apparatus can be improved.
Patent Document Japanese Patent Application Laid-Open No. 2001-53712
Non-Patent Document: “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” G. J. Foschini, Bell Labs Tech. J., pp. 41-59, Autumn 1996.